Calculation of equilibrium species for the aqueous solution systems of UO2SO4−U(SO4)2−H2SO4−HF and UO2Cl2−UCl4−HCl−HF at 298 K

Speciation models for aqueous solutions of UO₂SO₄−U(SO₄)₂−H₂SO₄−HF and UO₂Cl₂−UCl₄−HCl−HF were proposed based on chemical reaction equilibria, mass balances, charge balance, and stoichiometry of UF₄(s). The equilibrium concentrations of uranium and fluoride species in these solutions were calculated at 298 K, and are of relevance to the electrolytic reduction of U(VI), followed by the precipitation of UF₄(s). In these calculations, the reduction ratios of U(VI) were set at 25, 50, 75, and 100 pct. In the sulfate system the stable domains of U⁴⁺, U(SO₄) _n ⁴⁻²ⁿ , UF _n ⁴⁻ⁿ , and UF₄(s) as U(IV) species and UO ₂ ²⁺ , UO₂(SO₄) _n ²⁻²ⁿ , and UO₂F _n ²⁻ⁿ , as U(VI) species are strongly dependent on theC _T(F)/C _T(U(IV)) value. On the other hand, the stable domains of U⁴⁺ UCl³⁺, UF _n ^/4−n , and UF₄(s) as U(IV) species and UO ₂ ²⁺ , UO₂Cl⁺, and UO₂F _n ²⁻ⁿ as U(VI) species are also strongly affected by theC _T(F)/C _T(U(IV)) ratio in the chloride system. The initiation and precipitation of UF₄(s) in both the sulfate and chloride systems are a function of the reduction ratio of U(VI). The higher the reduction ratio, the lower theC _T(F)/C _T(U(IV)) values required. Compared to the chloride system, UF₄(s) precipitation in the sulfate system starts at a lower value ofC _T(F)/C _T(U(IV)). The addition of an excess amount of HF does not cause the dissolution of UF₄(s) precipitates because HF is a weak acid. KOJI SATO, formerly Graduate Student, Department of Metallurgy, Kyoto University, Kyoto, Japan